Our invention relates to the conversion of electric signals from analog to digital form. More specifically, our invention concerns a method of, and means for translating an analog audio or like data signal into an equivalent digital signal with the addition of dither (an artifically created white noise signal) to the analog input and subsequent removal of the dither from the digital output for the reduction of noise and distortion.
The pulse code modulation (PCM) or digital processing of audio signals has become, or is becoming, the mainstream of high fidelity sound recording and reproduction with the advent and ever increasing commercial acceptance of compact discs (CDs). There are, however, some problems left unsolved in the art of digital sound processing. One of these is the "quantization noise", that is, the difference between the samples of the music wave and the quantized values of the samples. The quantization noise becomes particularly pronounced, distorting the reproduced sound as higher harmonics, when the input signal level is low and there are only a small number of quantization steps. Even when the input signal level is high, the quantization noise will distort the signal if it changes slowly.
A conventional solution to this quantization noise problem has been the use of dither, intended to turn the quantization noise into white nolse which hardly effects the appreciation of the reproduced sound. An analog dither signal is superposed on the audio signal prior to its digitization and may, or may not, be removed from the digitized audio signal. This technique is disclosed for example in the article entitled "The Application of Large Amplitude Dither to the Quantization of Wide Range Audio Signals" by Yoshio Yamasaki in The Journal of the Acoustical Society of Japan. Vol 39, No. 7, published 1983, and in Japanese Laid Open Patent Application No. 50-68258.
We will now discuss two familiar examples of analog to digital conversion systems relying on dither for the suppression of quantization noise. One such known system adds an analog dither signal, converted from the output from a digital dither generator by a digital to analog (D/A) converter, to the incoming analog audio or data signal. The combined data and dither signal is then digitized by an analog to digital (A/D). Then the analog data and dither signal is fed to a subtracter circuit, to which is also supplied the digital dither signal directly from the dither generator for subtracting the dither component from the digital data and dither signal. We object to this known system not only because of the use of the D/A converter for transforming the digital dither signal but also because of the time difference between the dither fed directly from the dither generator to the substracter circuit and the dither delivered thereto with the data signal via the D/A converter, adder circuit, and A/D converter. This time difference makes impossible the complete removal of the dither component from the digitized data and digher signal because of the resulting phase difference between the dither fed directly from its generator and that which has been added to the data signal.
Another known system employs an analog dither generator in combination with a second A/D converter for digitizing the analog dither prior to its delivery to the subtracter. Although the analog dither put out by the generator can be fed directly to the adder circuit, the resulting dither added data signal is digitized by the first A/D converter, and the analog dither by the second A/D converter, preparatory to application to the two inputs of the subtracter circuit. We object to the use of the two A/D converters. Since no two A/D converters are absolutely alike in performance characteristics, they will give rise to different convertion errors, with the result that some dither component remain unremoved from the digital data signal to manifest itself as noise or distortion upon sound reproduction.